One example of a pixel circuit for an active type OLED display device is shown in FIG. 1. The source of a p-channel TFT1 for pixel drive is connected to a power supply PVdd, while the drain is connected to an anode 3 of an OLED (organic EL) element. Also, the cathode of the OLED element is connected to a low voltage power supply CV.
The gate of the TFT1 is connected to an auxiliary capacitor power supply Vcs via an auxiliary capacitor C, as well as to a data line Data for supplying a voltage based on pixel data (luminance data) via an n-channel TFT2 for selection. The gate of the TFT2 is connected to a gate line Gate extending in a horizontal direction.
At the time of display, the gate line is made H, and the TFT2 of a corresponding line is turned on. In this state pixel data is supplied to the data line Data, and this charges the auxiliary capacitor C. The TFT1 is then driven by a voltage in response to the pixel data, and the current through the TFT1 flows in the OLED element 3.
Here, the amount of emitted light and the current in the OLED element 3 has a substantially proportional relationship, but current begins to flow in the TFT1 when a voltage difference from the gate−PVdd exceeds a specified threshold voltage Vth. Pixel data supplied to the data line Data is added to the voltage (Vth) so that drain current starts to flow in the vicinity of image black level. Also, the amplitude of an image signal is such that there is specified luminance in the vicinity of a white level.
FIG. 2 is one example of a relationship between input voltage (Vgs), and luminance and current icv of the OLED element 3. In this manner, the OLED element 3 begins to emit light at voltage Vth, and is set such that emitted light becomes a white level at a white level input voltage Vw.
Here, since the luminance of light emitted from the OLED element 3 is proportional to the current flowing in the element, as described above, in order to drive a panel it is necessary to have a power supply capable of providing the current required when carrying out display for an image having maximum luminance over the entire surface. Accordingly, a power supply is required that has considerable surplus capacity compared to the power supply capacity required under normal usage conditions.
On the other hand, with a display device for displaying mainly natural images, used in a digital camera or video camera, the average level of image data is normally about 25%, and the maximum current of the power supply is rarely utilized.
There have also been proposed methods of calculating a histogram and average luminance in image frame units and generating panel drive data based on the results, with the intention of improving image quality and conserving power (see Japanese Patent Laid-open No. Hei. 7-322179 and Japanese Patent Laid-open No. 2002-116732.
With a display device having contrast and brightness adjustment built-in, it is necessary to determine the capacity of the power supply section assuming adjustment to the respective maximum values. There is a problem that the capacity of the power supply has to be significantly increased.